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<HR>

<H3>read_surf command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>read_surf filename keyword args ... 
</PRE>
<UL><LI>filename = name of surface file 

<LI>zero or more keyword/args pairs may be appended 

<LI>keyword = <I>origin</I> or <I>trans</I> or <I>atrans</I> or <I>ftrans</I> or <I>scale</I> or <I>rotate</I> or <I>transparent</I> or <I>invert</I> or <I>clip</I> or <I>group</I> or <I>typeadd</I> or <I>particle</I> or <I>file</I> 

<PRE>  origin args = Ox Oy Oz
    Ox,Oy,Oz = set origin of surface to this point (distance units)
  trans args = Dx Dy Dz
    Dx,Dy,Dz = translate origin by this displacement (distance units)
  atrans args = Ax Ay Az
    Ax,Ax,Az = translate origin to this absolute point (distance units)
  ftrans args = Fx Fy Fz
    Fx,Fy,Fz = translate origin to this fractional point in simulation box
  scale args = Sx Sy Sz
    Sx,Sy,Sz = scale surface by these factors around origin
  rotate args = theta Rx Ry Rz
    theta = rotate surface by this angle in counter-clockwise direction (degrees)
    Rx,Ry,Rz = rotate around vector starting at origin pointing in this direction
  transparent args = none
  invert args = none
  clip args = none or fraction
    fraction = push points close to the box boundary to the boundary (optional)
  group arg = group-ID
    group-ID = new or existing surface group to assign the surface elements to
  typeadd arg = Noffset
    Noffset = add Noffset to the type value of each element
  particle args = <I>none</I> or <I>check</I> or <I>keep</I>
    none = allow no particles in simulation when read surfs (default)
    check = delete particles inside surfs or in cells intersected by surfs
    keep = keep all particles
  file args = identical to those defined for the <A HREF = "write_surf.html">write_surf</A> command
    this keyword must be last 
</PRE>

</UL>
<P><B>Examples:</B>
</P>
<PRE>read_surf surf.sphere
read_surf surf.sphere group sphere2 typeadd 1
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip 1.0e-6
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip file tmp.surfs
read_surf surf.file trans 10 5 0 scale 3 3 3 invert clip file tmp.surfs.% points no nfile 32 
</PRE>
<P><B>Description:</B>
</P>
<P>Read the geometry of a surface from the specified file.  In SPARTA, a
"surface" is a collection of surface elements that represent the
surface(s) of one or more physical objects which will be embedded in
the global simulation box.  Surfaces can be explicit or implicit.
This command reads explicit surfaces from a file containing a list of
explicit surfaces.  See the <A HREF = "read_isurf.html">read_isurf</A> command to
read implicit surfaces from a different kind of file.  See the <A HREF = "Section_howto.html#howto_13">Howto
6.13</A> section of the manual for an
explantion of explicit versus implicit surfaces as well as distributed
versus non-distributed storage.  You cannot mix explicit and implicit
surfaces in the same simulation.
</P>
<P>Surface elements are triangles in 3d or line segments in 2d.  Surface
elements for each physical object are required to be a complete,
connected set that tile the entire surface of the object.  See the
discussion of watertight objects below.
</P>
<P>Particles collide with surface elements as they advect.  Each surface
element is assigned to a collision model, specified by the
<A HREF = "surf_collide.html">surf_collide</A> command which affects how a particle
bounces off the surface.  Each surface element can optionally be
assigned to a reaction model, specified by the
<A HREF = "surf_react.html">surf_react</A> command which determines if any surface
chemistry occurs during a collision.  Statistics for each surface
element due to their interactions with particles can be tallied via
the <A HREF = "compute_surf.html">compute surf</A> command, time-averaged via the
<A HREF = "fix_ave_surf.html">fix ave/surf</A> command, and ouput via the <A HREF = "dump.html">dump
surface</A> command.
</P>
<P>Surface elememts can be assigned to surface groups via the <A HREF = "group.html">group
surf</A> command.  Surface group IDs are used by other
commands to operate on selected sets of elements.  This command has
<I>group</I> and <I>typeadd</I> keywords which can be used to help assign
different elements or different objects to different groups.
</P>
<P>Explicit surface elements can be stored in a distributed fashion (each
processor only stores elements which overlap grid cells it owns or has
a ghost cell copy of).  Or each processor can store a copy of all
surface elements (the default).  See the <A HREF = "global.html">global surfs</A>
command to change this setting.
</P>
<P>Note that the read_surf command can be used multiple times to read
multiple objects from multiple files and add them to the simulation
domain.  The format of a surface file for explicit elements is
discussed below.  Optional keywords allow the vertices in the file to
be translated, scaled, and rotated in various ways.  This allows a
single surface file, e.g. containing a unit sphere, to be used
multiple times in a single simulation or in different simulations.
</P>
<P>The tools directory contains tools that can create surface files with
simple geometric objects (spheres, blocks, etc).  It also has tools
that can convert surface files in other formats to the SPARTA format
for explicit surfaces, e.g. for files created by a mesh-generation
program.
</P>
<HR>

<P>If all the surface elements are contained in a single file, the
specified file can be a text file or a gzipped text file (detected by
a .gz suffix).
</P>
<P>If a "%" character appears in the surface filename, SPARTA expects a
set of multiple files to exist.  The <A HREF = "write_surf.html">write_surf</A>
command explains how such sets are created.  Read_surf will first read
a filename where "%" is replaced by "base".  This file tells SPARTA
how many total surfaces and files are in the set (i.e. just the header
information described below).  The read_surf command then reads the
additional files.  For example, if the surface file was specified as
save.% when it was written, then read_surf reads the files save.base,
save.0, save.1, ... save.P-1, where P is the number of processors that
created the surface file.
</P>
<P>Note that P could be the total number of processors in the previous
simulation, or some subset of those processors, if the <I>fileper</I> or
<I>nfile</I> options were used when the surface file was written; see the
<A HREF = "write_surf.html">write_surf</A> command for details.  The processors in
the current SPARTA simulation share the work of reading these files;
each reads a roughly equal subset of the files.  The number of
processors which created the set can be different than the number of
processors in the current SPARTA simulation.  This can be a fast mode
of input on parallel machines that support parallel I/O.
</P>
<P>The remainder of this section describes the format of a single surface
file, whether it is the only file or one of multiple files flagged
with a processor number.
</P>
<P>A surface file for explicit surfaces has a header and a body.
The header appears first.  The first line of the header is always
skipped; it typically contains a description of the file.  Then lines
are read one at a time.  Lines can have a trailing comment starting
with '#' that is ignored.  If the line is blank (only whitespace after
comment is deleted), it is skipped.  If the line contains a header
keyword, the corresponding value is read from the line.  If it doesn't
contain a header keyword, the line begins the body of the file.
</P>
<P>The body of the file contains one or more sections.  The first line of
a section has only a keyword.  The next line is skipped.  The
remaining lines of the section contain values.  The number of lines in
a section depends on the section keyword as described below.  Zero or
more blank lines can be used between sections.  Sections can appear in
any order.
</P>
<P>The formatting of individual lines in the surface file (indentation,
spacing between words and numbers) is not important except that header
and section keywords must be capitalized as shown and can't have extra
white space between their words.
</P>
<P>These are the recognized header keywords.  Header lines can come in
any order.  The value(s) are read from the beginning of the line.
Thus the keyword <I>points</I> should be in a line like "1000 points".
</P>
<UL><LI><I>files</I> = # of files in set (only for base file, see below)
<LI><I>points</I> = # of points in surface (optional, see below)
<LI><I>lines</I> = # of line segments in surface (only allowed for 2d)
<LI><I>triangles</I> = # of triangles in surface (only allowed for 3d) 
</UL>
<P>The <I>files</I> keyword only appears in the "base" file for a set of
multiple files indicated by the "%" character in the filename.  It
tells SPARTA how many additional files exist in the set.  A "base"
file has no additional sections, i.e. no body.
</P>
<P>The <I>points</I> keyword is optional (see below).  For a set of multiple
files, it cannot appear in the "base" file, but only in individual
files in the set.
</P>
<P>The <I>points</I>, <I>lines</I>, <I>triangles</I> keywords refer to the number of
points, lines, triangles in an individual file.  Except in the case of
a "base" file for a set of multiple files.  In that case, the <I>lines</I>
and <I>triangles</I> keywords give the number of lines or triangles in the
entire set.
</P>
<P>These are the recognized section keywords for the body of the file.
</P>
<UL><LI><I>Points, Lines, Triangles</I> 
</UL>
<P>The <I>Points</I> section consists of N consecutive entries, where N = # of
points, each of this form:
</P>
<PRE>index x y z    (for 3d) 
index x y      (for 2d) 
</PRE>
<P>The index value is ignored; it is only added to assist in examining
the file.  When lines and triangles reference point indices they are
simply ordered from 1 to N, regardless of the actual value of the
index in the file.  <I>X,y,z</I> are the coordinates of the point in
distance units.  Note that for 2d simulations, <I>z</I> should be omitted.
</P>
<P>IMPORTANT NOTE: Unless points are on the surface of the simulation
box, they will be part of multiple lines or triangles.  However, there
is no requirement that each point appear exactly once in the <I>Points</I>
list.  For example, a point that is the common corner point of M
triangles, could appear 1 or 2 or up to M times.  However, if the same
point appears multiple times in the <I>Points</I> list, the coordinates of
all copies must be numerically identical, in order for SPARTA to
verify the surface is a watertight object, as discussed below.
</P>
<P>IMPORTANT NOTE: The <I>points</I> keyword and <I>Points</I> section are not
required.  You must either use both or neither.  As explained next, an
optional format for the <I>Lines</I> or <I>Triangles</I> sections includes point
coordinates directly with each line or triangle.
</P>
<P>The <I>Lines</I> section is only allowed for 2d simulations and consists of
N entries, where N = # of lines.  All entries must be in the same
format, either A or B.  If a Points section was included, use format
A.  If it was not, use format B.
</P>
<PRE>line-ID (type) p1 p2               # format A
line-ID (type) p1x p1y p2x p2y     # format B 
</PRE>
<P>The line-ID is stored internally with the line.  If the read_surf
commmand is reading a single file, the line-IDs should be unique
values from 1 to N where N is the number of lines specified in the
header of the file.  For a set of multiple files, each line in the
collection of all files should have a unique ID, and the IDs should
range from 1 to N, where N is the number of lines specified in the
base file.  SPARTA does not check line-IDs for uniqueness.  Note that
lines in an individual file (single or multiple) do not need to be
listed by ID order; they can be in any order.
</P>
<P>IMPORTANT NOTE: If the read_surf command is used when lines already
exist, i.e. to add new lines, then each line-ID is incremented by
Nprevious = the # of lines that already exist.
</P>
<P><I>Type</I> is an optional integer value which must be specified for all or
none of the lines in the file.  If used, it must be a positive integer
value for each line.  If not specified, the type of each line is set
to 1.  Line IDs and types can be used to assign lines to surface
groups via the <A HREF = "group.html">group surf</A> command.
</P>
<P>For format A, <I>p1</I> and <I>p2</I> are the indices of the 2 end points of the
line segment, as found in the Points section.  Each is a value from 1
to the # of points, as described above.  For format B, (p1x,p1y) and
(p2x,p2y) are the (x,y) coordinates of the two points (1,2) in the
line.
</P>
<P>The ordering of <I>p1</I>, <I>p2</I> is important as it defines the direction of
the outward normal for the line segment when a particle collides with
it.  Molecules only collide with the "outer" edge of a line segment.
This is defined by a right-hand rule.  The outward normal N = (0,0,1)
x (p2-p1).  In other words, a unit z-direction vector is crossed into
the vector from <I>p1</I> to <I>p2</I> to determine the normal.
</P>
<P>The <I>Triangles</I> section is only allowed for 3d simulations and
consists of N entries, where N = # of triangles.  All entries must be
in the same format, either A or B.  If a Points section was included,
use format A.  If it was not, use format B.
</P>
<PRE>tri-ID (type) p1 p2 p3                                  # format A
tri-ID (type) p1x p1y p1z p2x p2y p2z p3x p3y p3z       # format B 
</PRE>
<P>The tri-ID is stored internally with the line.  If the read_surf
commmand is reading a single file, the tri-IDs should be unique values
from 1 to N where N is the number of triangles specified in the header
of the file.  For a set of multiple files, each triangle in the
collection of all files should have a unique ID, and the IDs should
range from 1 to N, where N is the number of triangles specified in the
base file.  SPARTA does not check tri-IDs for uniqueness.  Note that
triangles in an individual file (single or multiple) do not need to be
listed by ID order; they can be in any order.
</P>
<P>IMPORTANT NOTE: If the read_surf command is used when triangles
already exist, i.e. to add new triangles, then each tri-ID is
incremented by Nprevious = the # of triangles that already exist.
</P>
<P><I>Type</I> is an optional integer value which must be specified for all or
none of the triangles in the file.  If used, it must be a positive
integer value for each triangle.  If not specified, the type of each
triangle is set to 1.  Triangle IDs and types can be used to assign
triangles to surface groups via the <A HREF = "group.html">group surf</A> command.
</P>
<P>For format A, <I>p1</I>, <I>p2</I>, and <I>p3</I> are the indices of the 3 corner
points of the triangle, as found in the Points section.  Each is a
value from 1 to the # of points, as described above.  For format B,
(p1x,p1y,p1z), (p2x,p2y,p2z), and (p3x,p3y,p3z) are the (x,y,z)
coordinates of the three corner points (1,2,3) of the triangle.
</P>
<P>The ordering of <I>p1</I>, <I>p2</I>, <I>p3</I> is important as it defines the
direction of the outward normal for the triangle when a particle
collides with it.  Molecules only collide with the "outer" face of a
triangle. This is defined by a right-hand rule.  The outward normal N
= (p2-p1) x (p3-p1).  In other words, the edge from <I>p1</I> to <I>p2</I> is
crossed into the edge from <I>p1</I> to <I>p3</I> to determine the normal.
</P>
<HR>

<P>The following optional keywords affect the geometry of the read-in
surface elements.  The geometric transformations they describe are
performed in the order they are listed, which gives flexibility in how
surfaces can be manipulated.  Note that the order may be important;
e.g. performing an <I>origin</I> operation followed by a <I>rotate</I> operation
may not be the same as a <I>rotate</I> operation followed by an <I>origin</I>
operation.
</P>
<P>Most of the keywords perform a geometric transformation on all the
vertices in the surface file with respect to an origin point.  By
default the origin is (0.0,0.0,0.0), regardless of the position of
individual vertices in the file.
</P>
<P>The <I>origin</I> keyword resets the origin to the specified <I>Ox,Oy,Oz</I>.
This operation has no effect on the vertices.
</P>
<P>The <I>trans</I> keyword shifts or displaces the origin by the vector
(Dx,Dy,Dz).  It also displaces each vertex by (Dx,Dy,Dz).
</P>
<P>The <I>atrans</I> keyword resets the origin to an absolute point (Ax,Ay,Az)
which implies a displacement (Dx,Dy,Dz) from the current origin.  It
also displaces each vertex by (Dx,Dy,Dz).
</P>
<P>The <I>ftrans</I> keyword resets the origin to a fractional point
(Fx,Fy,Fz).  Fractional means that Fx = 0.0 is the lower edge/face in
the x-dimension and Fx = 1.0 is the upper edge/face in the
x-dimension, and similarly for Fy and Fz.  This change of origin
implies a displacement (Dx,Dy,Dz) from the current origin.  This
operation also displaces each vertex by (Dx,Dy,Dz).
</P>
<P>The <I>scale</I> keyword does not change the origin.  It computes the
displacement vector of each vertex from the origin (delx,dely,delz)
and scales that vector by (Sx,Sy,Sz), so that the new vertex
coordinate is (Ox + Sx*delx,Oy + Sy*dely,Oz + Sz*delz).
</P>
<P>The <I>rotate</I> keyword does not change the origin.  It rotates the
coordinates of all vertices by an angle <I>theta</I> in a counter-clockwise
direction, around the vector starting at the origin and pointing in
the direction <I>Rx,Ry,Rz</I>.  Any rotation can be represented by an
appropriate choice of origin, <I>theta</I> and (Rx,Ry,Rz).
</P>
<P>The <I>transparent</I> keyword flags all the read in surface elements as
transparent, meaning particles pass through them.  This is useful for
tallying flow statistics.  The <A HREF = "surf_collide.html">surf_collide
transparent</A> command must also be used to assign a
transparent collision model to those the surface elements.  The
<A HREF = "compute_surf.html">compute surf</A> command will tally fluxes differently
for transparent surf elements.  The <A HREF = "Section_howto.html#howto_15">Section
6.15</A> doc page provides an overview of
transparent surfaces.  See those doc pages for details.
</P>
<P>The <I>invert</I> keyword does not change the origin or any vertex
coordinates.  It flips the direction of the outward surface normal of
each surface element by changing the ordering of its vertices.  Since
particles only collide with the outer surface of a surface element,
this is a mechanism for using a surface files containing a single
sphere (for example) as either a sphere to embed in a flow field, or a
spherical outer boundary containing the flow.
</P>
<P>The <I>clip</I> keyword does not change the origin.  It truncates or
"clips" a surface that extends outside the simulation box in the
following manner.  In 3d, each of the 6 clip planes represented by
faces of the global simulation box are considered in turn.  Any
triangle that straddles the face (with points on both sides of the
clip plane), is truncated at the plane.  New points along the edges
that cross the plane are created.  A triangle may also become a
trapezoid, in which case it turned into 2 triangles.  Then all the
points on the side of the clip plane that is outside the box, are
projected onto the clip plane.  Finally, all triangles that lie in the
clip plane are removed, as are any points that are unused after the
triangle removal.  After this operation is repeated for all 6 faces,
the remaining surface is entirely inside the simulation box, though
some of its triangles may include points on the faces of the
simulation box.  A similar operation is performed in 2d with the 4
clip edges represented by the edges of the global simulation box.
</P>
<P>IMPORTANT NOTE: If a surface you clip crosses a periodic boundary, as
specified by the <A HREF = "boundary.html">boundary</A> command, then the clipping
that takes place must be consistent on both the low and high end of
the box (in the periodic dimension).  This means any point on the
boundary that is generated by the clip operation should be generated
twice, once on the low side of the box and once on the high side.  And
those two points must be periodic images of each other, as implied by
periodicity.  If the surface you are reading does not clip in this
manner, then SPARTA will likely generate an error about mis-matched or
inconsistent cells when it attempts to mark all the grid cells and
their corner points as inside vs outside the surface.
</P>
<P>If you use the <I>clip</I> keyword, you should check the resulting
statistics of the clipped surface printed out by this command,
including the minimum size of line and triangle edge lengths.  It is
possible that very short lines or very small triangles will be created
near the box surface due to the clipping operation, depending on the
coordinates of the initial unclipped points.
</P>
<P>If this is the case, an optional <I>fraction</I> argument can be appended
to the <I>clip</I> keyword.  <I>Fraction</I> is a unitless value which is
converted to a distance <I>delta</I> in each dimension where delta =
fraction * (boxhi - boxlo).  If a point is nearer than delta to the lo
or hi boundary in a dimension, the point is moved to be on the
boundary, before the clipping operation takes place.  This can prevent
tiny surface elements from being created due to clipping.  If
<I>fraction</I> is not specified, the default value is 0.0, which means
points are not moved.  If specified, <I>fraction</I> must be a value
between 0.0 and 0.5.
</P>
<P>Note that the <I>clip</I> operation may delete some surface elements and
create new ones.  Likewise for the points that define the end points
or corner points of surface element lines (2d) or triangles (3d).  The
resulting altered set of surface elements can be written out to a file
by the <A HREF = "write_surf.html">write_surf</A> command, which can then be used an
input to a new simulation or for post-processing and visualization.
</P>
<P>IMPORTANT NOTE: When the <I>clip</I> operation deletes or adds surface
elements, the line-IDs or tri-IDs will be renumbered to produce IDs
that are consective values from 1 to the # of surface elements.  The
ID of a surface element that is unclipped may change due to this
reordering.
</P>
<HR>

<P>The following optional keywords affect group and type settings for the
read-in surface elements and output of the elements.  Also how
particles are treated when surface elements are added.
</P>
<P>Surface groups are collections of surface elements.  Each surface
element belongs to one or more surface groups; all elements belong to
the "all" group, which is created by default.  Surface group IDs are
used by other commands to identify a group of suface elements to
operate on.  See the <A HREF = "group.html">group surf</A> command for more details.
</P>
<P>Every surface element also stores a <I>type</I> which is a positive
integer.  <I>Type</I> values are useful for flagging subsets of elements or
different objects in the surface file.  For example, a patch of
triangles on a sphere.  Or one sphere out of several that the file
contains.  Surface element types can be used to define surface groups.
See the <A HREF = "group.html">group surf</A> command for details.
</P>
<P>The <I>group</I> keyword specifies an extra surface <I>group-ID</I> to assign
all the read-in surface elements to.  All the read-in elements are
assigned to the "all" group and to <I>group-ID</I>.  If <I>group-ID</I> does not
exist, a new surface group is created.  If it does exist the read-in
surface elements are added to that group.
</P>
<P>The <I>typeadd</I> keyword defines an <I>Noffset</I> value which is added to the
type of each read-in surface element.  The default is Noffset = 0,
which means the read-in type values are not altered.  If type values
are not included in the file, they default to 1 for every element, but
can still be altered by the <I>typeadd</I> keyword.
</P>
<P>Note that use of the <I>group</I> and <I>typeadd</I> keywords allow the same
surface file to be read multiple times (e.g. with different origins,
tranlations, rotations, etc) to define multiple objects, and assign
their surface elements to different groups or different type values.
</P>
<P>The <I>particle</I> keyword determines how particles in the simulation are
affected by the new surface elements.  If the setting is <I>none</I>, which
is the default, then no particles can exist in the simulation.  If the
setting is <I>check</I>, then particles in grid cells that are inside the
new watertight surface object(s) or in grid cells intersected by the
new surface elements are deleted.  This is to insure no particles will
end up inside a surface object, which will typically generate errors
when particles move.  If the setting is <I>keep</I> then no particles are
deleted.  It is up to you to insure that no particles are inside
surface object(s), else an error may occur later.  This setting can
be useful if a <A HREF = "remove_surf.html">remove_surf</A> was used to remove
a surface object, and a new object is being read in, and you know
the new object is smaller than the one it replaced.  E.g. for a model
of a shrinking or ablating object.
</P>
<P>If the <I>file</I> keyword is used, the surfaces will be written out to the
specified <I>filename</I> immediately after they are read in.  The
arguments for this keyword are identical to those used for the
<A HREF = "write_surf.html">write_surf</A> command.  This includes a file name with
optional "*" and "%" wildcard characters, as well as its optional
keywords.
</P>
<P>IMPORTANT NOTE: The <I>file</I> keyword must be the last keyword specified
with the read_isurf command.  This is because all the remaining arguments
are passed to the <A HREF = "write_surf.html">write_surf</A> command.
</P>
<P>The format for the output file is the same as the one written by the
<A HREF = "write_surf.html">write_surf</A> command, or read by this command.  Note
that it can be useful to write out a new surface file after reading
one if clipping was performed; the new file will contain the surface
element altered by clipping and will not contain any surface elements
removed by clipping.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>This command can only be used after the simulation box is defined by
the <A HREF = "create_box.html">create_box</A> command, and after a grid has been
created by the <A HREF = "create_grid.html">create_grid</A> command.  If particles
already exist in the simulation, you must insure particles do not
end up inside the added surfaces.  See the <I>particle</I> keyword
for options with regard to particles.
</P>
<P>To read gzipped surface files, you must compile SPARTA with the
-DSPARTA_GZIP option - see <A HREF = "Section_start.html#start_2">Section 2.2</A> of
the manual for details.
</P>
<P>The <I>clip</I> keyword cannot be used when the <A HREF = "global.html">global surfs
explicit/distributed</A> command has been used.  This is
because we have not yet figured out how to clip distributed surfaces.
</P>
<P>Every vertex in the final surface (after translation, rotation,
scaling, etc) must be inside or on the surface of the global
simulation box.  Note that using the <I>clip</I> operation guarantees that
this will be the case.
</P>
<P>The surface elements in a single surface file must represent a
"watertight" surface.  For a 2d simulation this means that every point
is part of exactly 2 line segments.  For a 3d simulation it means that
every triangle edge is part of exactly 2 triangles.  Exceptions to
these rules allow for triangle edges (in 3d) that lie entirely in a
global face of the simulation box, or for line points (in 2d) that are
on a global edge of the simulation box.  This can be the case after
clipping, which allows for use of watertight surface object (e.g. a
sphere) that is only partially inside the simulation box, but which
when clipped to the box becomes non-watertight, e.g. half of a sphere.
</P>
<P>Note that this definition of watertight does not require that the
surface elements in a file represent a single physical object;
multiple objects (e.g. spheres) can be represented, provided each is
watertight.
</P>
<P>Another restriction on surfaces is that they do not represent an
object that is "infinitely thin", so that two sides of the same object
lie in the same plane (3d) or on the same line (2d).  This will not
generate an error when the surface file is read, assuming the
watertight rule is followed.  However when particles collide with the
surface, errors will be generated if a particle hits the "inside" of a
surface element before hitting the "outside" of another element.  This
can occur for infinitely thin surfaces due to numeric round-off.
</P>
<P>When running a simulation with multiple objects, read from one or more
surface files, you should insure they do not touch or overlap with
each other.  SPARTA does not check for this, but it will typically
lead to unphysical particle dynamics.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "read_isurf.html">read_isurf</A>, <A HREF = "write_surf.html">write_surf</A>
</P>
<P><B>Default:</B>
</P>
<P>The default origin for the vertices in the surface file is (0,0,0).
The defaults for group = all, type = no, toffset = 0, particle = none.
</P>
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